The current fossil fuel-based energy systems, though meeting the energy needs of economic and social development, have incurred severe regional and global environmental issues. As a clean energy, hydrogen generates only water while utilized, without causing pollutions to environments. However, approximately 96% of the current hydrogen stems from fossil energy, among which 48% comes from natural gas, 30% from tail gases of refineries and chemical plants, and 18% from coal. Besides, the principal emission CO2 generated during the conversion process has not been effectively captured (IEA, 2007). This leads to the situation where those existing fossil-based hydrogen energy systems will still exacerbate the greenhouse effect. At the same time, fossil energy is a non-renewable one that likewise will face its depletion in the near future.
As a carbon neutral renewable energy, biomass is very widespread and in great abundance. Generating hydrogen from biomass, on the one hand, enables an efficient utilization of biomass resources, and on the other hand can attain renewable low-carbon hydrogen to ensure that hydrogen energy systems are featured by environmental friendliness and sustainability.
At present, the technologies of generating hydrogen from biomass are mainly divided into two categories: one involves biological methods in which hydrogen is generated in a biological fashion, e.g. photo-fermentation, anaerobic fermentation, biophotolysis of water, biological photo-fermentation and the like; and the other category refers to thermochemical methods in which hydrogen generation is done by thermochemical reactions of the raw material biomass, such as hydrogen generation by pyrolysis/gasification, hydrogen generation by supercritical conversion, hydrogen generation by plasma gasification, etc. These two categories of methods have their characteristics and have been greatly developed for the past few years. But in general, hydrogen generation by biological methods is mostly suitable for medium- and small-scale applications due to its relatively stringent biological growth conditions and slower reaction rates, whereas both hydrogen generation by supercritical conversion and hydrogen generation by plasma gasification necessitate high temperatures during their reactions, and this consequently lead to unacceptable energy consumptions. As a result, further development needs to be made for all these technologies. Hydrogen generation by biomass pyrolysis/gasification, however, can draw lessons from those comparatively well-developed coal gasification technologies, and thus has high reaction rate and relatively moderate reaction conditions and also is easy to realize mass generation, making itself highly promising in the field of hydrogen generation from biomass, especially in the field of hydrogen generation from biomass that is hardly degradable.
The conventional procedure for hydrogen generation by biomass pyrolysis/gasification is to convert hydrocarbons in biomass into combustible components, e.g. CO, H2, CH4 and tar, under certain thermochemical conditions, then further convert macromolecular substances, such as tar, in the gasification product into small molecule gases through catalytic cracking and convert CO, CH4 and other such gases in the cracked product into H2 and CO2 through steam reforming, and finally subject the resulting gas to purification, decarburization and other such processes to produce high-purity hydrogen. This technical route of gasification, cracking, reforming, purification and decarburization has made it possible to generate high-purity hydrogen from biomass. But since it has a considerable length, numerous disadvantages arise, such as, increased operational complexity, higher energy consumption and a substantial increase in hydrogen generation costs.